1. Technical Field
The invention is in the field of electro-optic modulators of light wave direction.
2. Background
Laser beam steering or scanning technology is important for many optical systems including imaging laser radars, projection displays, optical interconnections, optical switches, and laser printers. Although several non-mechanical approaches to beam steering or scanning have been demonstrated, only a few have been adopted in practical systems. Beam scanning with rotating mirrors has been by far the most widely used technology, L. Bieser, et al, Handbook of Optics, M. Bass, ed., vol. 2, Ch. 19 (Optical Society of America, McGraw-Hill, New York, 1994). However, rotating mirror scanners are not suitable for compact optical systems. In addition, rotating mirror scanners may be vulnerable to vibrations and accelerations. Although significant advances have been made in the development of micro-mirror beam deflectors, currently available devices can only provide binary beam steering with moderate speed.
After mechanical mirrors, the next most widely used are acousto-optic deflectors, A. Korpel, Applied Optics and Optical Engineering, R. Kingslake, ed., vol. 6, (Academic Press, New York, 1980). These are bulk devices which are generally costly because of the high driving voltages required. Other devices include binary and ferroelectric liquid crystal beam deflectors, K. Hirabayashi et al, Applied Optics, vol.34, pp 2571 et seq (1995). At present, liquid crystal beam steering devices can provide reasonable performance for polarized light in temperature-controlled environments. However, their use in systems subject to large temperature fluctuations has been hindered because of the significant degradation of their electro-optical properties at very low or very high ambient temperatures and their response times are slow.
For some time, L. Bieser et al, SPIE Milestone Series, vol. 378 (1985), a large effort has been made to overcome the limitations of mechanical, acousto-optic and liquid crystal based beam deflectors using solid electro-optic crystals. These are characterized as having an index of refraction which changes in response to an electric field. The change with electric field can be linear (Pockels Effect) or quadratic (Kerr Effect). There are a number of different schemes which have been proposed. Many of these are based on the principle that, if a coherent light beam passes through a material and experiences a linear variation in index of refraction (equivalently, the velocity of light in the medium varies linearly) across the face of the beam, it will be bent in the direction of increasing index. U.S. Pat. No. 3,357,771 to Buhrer et al. discloses a beam deflector comprising an elongated bulk crystal of linear electro-optic effect potassium dihydrogen phosphate encased on two opposite elongated sides between hyperbolically shaped dielectric blocks which are in turn coated with conductors such that, when an AC voltage is applied to the conductors, a linear electric field variation occurs in the crystal causing the necessary linear variation in refractive index. A light beam propagating along the length of the crystal transverse to the electric field is deflected toward either of the non-encased elongated sides depending on instantaneous voltage polarity. U.S. Pat. No. 5,159,351 to Hotomi et al. using a lead lanthanum zirconate titanate (PLZT) bulk crystal relies on either a trapezoidal cross section coated on its even sides with a conductor or a rectangular cross section coated on one side with a resistive sheet electrode to achieve almost the same effect. Because the distance between electrodes is not small, it is not possible to create a perfectly linear variation. Also, in this bulk approach, the driving voltage is reasonably large (150 volts for a 2.34 degree deflection).
Another approach, U.S. Pat. No. 4,343.536 to Watanabe et al., uses a dual array of interdigitated stripe electrodes on one surface of an elongated crystal. When an AC voltage, especially in the MHz frequency range is applied, fringing fields extend into the interior and, even though not precisely linear, can cause a controllable deflection of a light beam. The patented device works with a large number of materials and is incorporated herein by reference for such teachings.
The aforementioned devices in which an electric field is applied transversely to the light propagation direction is advantageous in that the longer the device, the more the total deflection for a given driving voltage. The disadvantage is that microminiaturization is difficult and they are polarization sensitive.
For larger beam diameters, U.S. Pat. No. 3,787,111 to Giordmaine et al. discloses a device comprising a layer of strontium barium niobate (SBN) on a transparent substrate. A closely-spaced array of linear stripe electrodes is deposited on the SBN. Independent voltage means are used to change the index of refraction under each electrode creating, along a direction perpendicular to the stripes, an approximation to a linear variation in index of refraction. A light beam whose diameter is large compared to the electrode spacing traveling through the substrate and SBN will be deflected in a direction perpendicular to the stripes. In order to be most successful, this approach requires an electrode spacing which is small compared to the wavelength of the light. See U.S. Pat. No. 5,093,747 to Dorschner. If the electrode spacing is not small, diffraction effects occur creating side lobes off the main beam. This effect was utilized to advantage in U.S. Pat. No. 4,639,091 to Huignard et al.
In addition to light transmitted through a layered device, beam deflection of a reflected beam is desirable, especially for spatial light modulators (SLMs). U.S. Pat. No. 5,221,989 to Stappaerts et al. discloses a device utilizing a non-ferroelectric PLZT (obtained when the lanthanum concentration is in the range of 9% to 10%) ceramic plate coated on the light incident side (hereinafter "front") with a conducting semi-transparent electrode and on the back side with a rectangular array of reflecting electrode islands which may be further electrically isolated by placing grooves between them in the PLZT plate. For SLMs, the beam is not only deflected but intentionally distorted by producing desired phase shifts in each part of a light beam corresponding the to the size of the islands in the referenced patent. This device can utilize the linear Pockels effect, but utilizing the quadratic Kerr effect with a DC bias voltage requires less modulation voltage. For example, a bias of 550 volts allowed the use of a modulation voltage of 100 volts to produce a .pi./2 phase shift. By using a semi-transparent front electrode with a reflectance of 30%, the modulation voltage could be reduced by one third because of optical resonance between the front and back electrodes. However in this device, to minimize the side lobes due to diffraction effects, both the beam diameter and wavelength must be substantially larger than an individual pixel size, typically limited to no smaller than about one micron.
In summary, bulk crystal-based electro-optic deflectors, have not yet overcome the difficulties associated with their high driving voltages. Beam steering using multi-channel phased arrays has significant drawbacks because of the presence of higher-order deflections.
Recently, there has been great interest in developing organic electro-optic materials. Although they tend to be unstable with time and temperature, some show large electro-optic coefficients and they can be produced as thin films. See U.S. Pat. Nos. 4,773,743; 4,783,151; 4,807,968; 4,885,113; 4,887,889; 4,909,964; 4,962,979; 4,035,839; 5,053,168; 5,061,048; 5,112,532; 5,185,102; 5,194,548; 5,194,984; 5,200,541; 5,256,784; 5,323,482; 5,432,286; and 5,157,541 all of which are incorporated herein by reference. These patents indicate that many organic materials, if they have non-centrosymmetric crystalline structures, can produce a useful Pockels effect but these patents present only very basic devices.
Another application area of significant interest is optical interconnects in free-space routing. Current technologies, including multiple quantum well based electro-absorption modulators based on GaAs and related materials and surface emitting micro-lasers, have encountered substantial difficulties because of their material incompatibility with silicon-based microelectronic devices.